A screengrab gives you the screen, nothing more. if you need high resolution you have to display the image at high resolution. If the high resolution image exceeds the size of the screen a screengrab will give you a partial image. If you scroll the screen around you can get screengrabs of different parts of the image and combine them in Photoshop (or other software).

Some browsers will take a high resolution image and reduce it to fit the screen. If your browser does that it will change the cursor to a circle with a + inside. Clicking on the image will give you the full resolution, larger than the screen. But on a browser, a right-click will sometimes enable you to download the high resolution image. Some browsers enable the web page to overlay the displayed image with a blank image such that it doesn't show on the screen but you get the blank image when you download it. The screengrab is immune to that sort of protection.

It sounds to me as if you are looking for a sharing site rather than a storage site. I have never used imgur but I use flickr and it's pretty easy for someone to download an image.

First of all, if you (or someone else) can display an image on a monitor you (or someone else) can grab a copy of it no matter what program is displaying it.

Here's a photo from flickr.

https://static.uglyhedgehog.com/upload/2023/5/4/t1-665583-flickr_2023_05_04_18.24.32.jpg

The red arrow points to the download function. Clicking on that will bring up a dialog allowing you to choose the size of the downloaded file. Selecting a size will bring up a download dialog which will allow you to navigate to the folder you want the image sent to. Using Win10 for this example, but it's similar on a Mac.

If your image is not in flickr, displaying it on your monitor will allow you to download it by a screengrab. Most browsers support some sort of screen grab software that allows you to make a copy of the image being displayed on the screen and store the copy somewhere in your computer. Details will depend on the software used. Some programs can be used in the operating system and are not dependent on a browser. Google is your friend.

I have no obection to your using the above image for testing screen grab software. Or if you want to check out flickr, the image comes from https://www.flickr.com/photos/photofarmer/48790332702/