What size do images need to be for them to be successfully uploaded by users?

Our form requires artists to attach 3 images. I am getting feedback that they don't know what size the images should be (max and min).

The file upload sizing depends on what you set.  The maximum file upload size can be increased.  Here is our quick guide detailing how to increase the limit as well as the maximum file sizes for different integrations:

https://www.jotform.com/help/33-Changing-the-Upload-Size-Limit

You can also change the allowed file types:

https://www.jotform.com/help/29-Upload-Field-Allowed-File-Types