A couple of thoughts:

1) Use virtual copies in your workflow. I mean develop the image in the original dimension / aspect ratio. As a final step, crop the image for posting and display. I'll assume 2x3 for purposes of this discussion. Maybe the composition is better cropped tighter. As long as the edits are all done in LR, the cropping is virtual and can be done at any point. But, if you pass the image to a plug-in, you should crop the results after working in the plug-in, not before.

2) Should a need come for another aspect, such as 4x5, make a LR virtual copy of the posted image and then edit the copy. All of your edits are available in the virtual copy including changing the cropping. You now have two (or as many as needed) copies of the image in the LR catalog, all developed from the single original image.

A couple of thoughts:

1) Use virtual copies in your workflow. I mean develop the image in the original dimension / aspect ratio. As a final step, crop the image for posting and display. I'll assume 2x3 for purposes of this discussion. Maybe the composition is better cropped tighter. As long as the edits are all done in LR, the cropping is virtual and can be done at any point. But, if you pass the image to a plug-in, you should crop the results after working in the plug-in, not before.

2) Should a need come for another aspect, such as 4x5, make a LR virtual copy of the posted image and then edit the copy. All of your edits are available in the virtual copy including changing the cropping. You now have two (or as many as needed) copies of the image in the LR catalog, all developed from the single original image.

A couple of thoughts:

1) Use virtual copies in your workflow. I mean develop the image in the original dimension / aspect ratio. As a final step, crop the image for posting and display. I'll assume 2x3 for purposes of this discussion. Maybe the composition is better cropped tighter. As long as the edits are all done in LR, the cropping is virtual and can be done at any point. But, if you pass the image to a plug-in, you should crop the results after working in the plug-in, not before.

2) Should a need come for another aspect, such as 4x5, make a LR virtual copy of the posted image and then edit the copy. All of your edits are available in the virtual copy including changing the cropping. You now have two (or as many as needed) copies of the image in the LR catalog, all developed from the single original image.

You CAN post a single image, but each aspect ratio will crop it differently, usually based on the center of the file you send. For instance, a typical dSLR 2:3 aspect ratio image will make an un-cropped 8x12 or 4x6, but you lose an inch off each end when cropped to 8x10. Maybe you want to lose TWO inches at one end and keep what's on the other...

I usually crop my images as I want them, and only offer sizes that match. Yes, that means making certain images available ONLY at certain sizes.

My favorite compositions are 1:1, 2:3, 3:4, 4:5, 5:7, 11:14, and 16:9. (4x4, 8x8; 4x6, 8x12, 12x18, 24x36; 6x8, 9x12, 12x16, 24x32, 30x40; 4x5, 8x10, 16x20, 32x40; 5x7 (the oddball); 11x14 (another oddball); and 16x9, 8x4.5, 32x18. All but the squares can be horizontal or vertical. Those last three are usually horizontal compositions, although 9x16 can work for full length portraits. There is a 10x13 size that I find ridiculous. We offered it as a bonus print in the school portrait industry, and in the other mass portrait markets, but it is hard to find frames and the crop is weird for portraits.

I find myself drawn more and more to the 16:9 because of HDTV and 4K screens. My camera can process JPEGs to 1:1, 3:2, 4:3, and 16:9, so I often record both raw and JPEG files in one of these crop modes. That ensures I have the full 4:3/3:4 area of my sensor on record (the raw file), but I have framed it in the viewfinder to fit one of the other sizes. I can process the raw any way I want, but I've preserved my original composition in the JPEG.

In the school picture industry, we always had cropping masks or guidelines in the viewfinders of our cameras. I was the guy who designed and ordered the masks for our photographers. Here's a link to a company that makes both stock and custom viewfinder masks:

http://www.viewfindermasks.com (this site requires Adobe Flash).

One problem you encounter with many photo labs is that they force your picture to fill a print. If you order an 8x10 and the aspect ratio of your photo is not 4:5 the lab will take the side that's too short and fill the resulting print with it, thereby chopping off some of the longer side.

If the aspect ratio of your photo is not the same as the aspect ratio of the print you are ordering you will want to pad the short side with white space so that the border will be a bit wider on the resulting print (or there will be a white border on two sides if you order the print without borders). Many programs exist that will enable you to change the size of the image by adding white space. Some programs will allow you to add some other color (maybe black space). In Photoshop it's pretty easy, but if you don't have Photoshop, you can do it in Microsoft Paint (which comes free with Windows). If you're a Mac person, there must be similar programs.