I propose two guidelines.

1. Crop out unneeded info: If you take a full screen shot of your desktop to show an error message, just crop the picture in paint or something to show the relevant error message. We don't really need to see all the stuff in the background, and if you aren't paying attention, you may accidentally include information about yourself or your accounts that you did not want to include. If you see someone who didn't crop their photo, you can copy the image url and crop it with this online tool and reupload the image into the post (I am not affiliated with this tool in anyway). Also, if you notice that there is PII or account information that could potentially compromise the users security or privacy edit the information out and then flag for moderator attention to purge it from the edit history.
2. Resize images to a reasonable size: If you upload a picture through stack exchange, it uses the Imgur engine. This means you can easily resize the images to a more manageable size so that they do not take up the entire screen. There are a number of different suffixes you can use to resize the image by using the form https://i.sstatic.net/XXXXXXsuffix.jpg where the suffix is one of the following:

• h - Huge thumbnail - size: 1024x1024 and keeps image proportions.
• l - Large thumbnail - size: 640x640 and keeps image proportions.
• m - Medium thumbnail - size: 320x320 and keeps image proportions.
• t - Small thumbnail - size: 160x160 and keeps image proportions.
• b - Big square thumbnail - size: 160x160 doesn't keep image proportions.
• s - Small square thumbnail - size: 90x90 doesn't keep image proportions.

Large or medium thumbnail will generally give you a decent size image in proportion to page size while still allowing you to see everything you need. You can see examples of each suffix in comparison to the original here. If you don't want to use suffixes, you could use a more complex method with using HTML like so: <img src="https://i.sstatic.net/XXXXXX.jpg" width="100" height="100">

I propose two guidelines.

1. Crop out unneeded info: If you take a full screen shot of your desktop to show an error message, just crop the picture in paint or something to show the relevant error message. We don't really need to see all the stuff in the background, and if you aren't paying attention, you may accidentally include information about yourself or your accounts that you did not want to include. If you see someone who didn't crop their photo, you can copy the image url and crop it with this online tool and reupload the image into the post (I am not affiliated with this tool in anyway). Also, if you notice that there is PII or account information that could potentially compromise the users security or privacy edit the information out and then flag for moderator attention to purge it from the edit history.
2. Resize images to a reasonable size: If you upload a picture through stack exchange, it uses the Imgur engine. This means you can easily resize the images to a more manageable size so that they do not take up the entire screen. There are a number of different suffixes you can use to resize the image by using the form https://i.sstatic.net/XXXXXXsuffix.jpg where the suffix is one of the following:

• h - Huge thumbnail - size: 1024x1024 and keeps image proportions.
• l - Large thumbnail - size: 640x640 and keeps image proportions.
• m - Medium thumbnail - size: 320x320 and keeps image proportions.
• t - Small thumbnail - size: 160x160 and keeps image proportions.
• b - Big square thumbnail - size: 160x160 doesn't keep image proportions.
• s - Small square thumbnail - size: 90x90 doesn't keep image proportions.

Large or medium thumbnail will generally give you a decent size image in proportion to page size while still allowing you to see everything you need. You can see examples of each suffix in comparison to the original here. If you don't want to use suffixes, you could use a more complex method with using HTML like so: <img src="https://i.sstatic.net/XXXXXX.jpg" width="100" height="100">

Also as mentioned by Robotnik, "image links are now auto-surrounded by a hyperlink to expand the image out. You can manually change the hyperlink to point to a larger image. Where you have [![alt text][1]][1], change it to [![alt text][1]][2], then add a [2]: link at the bottom pointing to the larger version."