This is from the help files.  The button to open the image appears to use the workstation's default application for the file so I don't think you have to be concerned about the dimensions unless you intend to include the image in reports.

Image

Enter an image file to associate with this item, or click the Browse File button to browse for an image file. Click the Show Image button to preview the image file. Only image files with an extension of .bmp, .jpg, or .gif are accepted (.jpg or .gif files are recommended). Inventory item images are stored in the MAS90\Images\xxx folder (where xxx is the company code) on the server and published to the Internet Information Services (IIS). Because all image files reside at the same Internet Information Services location, image file names must be unique system wide.

Warning When .bmp images are used on a Web page, performance is extremely slow.

If you are using Sage 100 Advanced, to use the Image feature, you must have entered a UNC path in the Sage 100 Advanced Server field during Workstation Setup.