Resizing an image with GIMP.

Given three requirements:
1. Maximum dimensions (width × height)
2. Minimum dimensions (width × height)
3. Maximum file size (megabytes)

We assume that the file already meets the minimum dimensions (else you will
need to take another photo). The procedure is this:
1. Load the image. (If it suggests rotating, do this.)
2. Crop if necessary (Image→Canvas Size).
2. Export as jpeg (File→Export), as is. See by how much it is too large. If not too big, fine. If too big, see by how much, and scale down.
2. Scale the image (Image→Scale Image) to get the dimensions and file size right. Repeat this step as necessary. First try as large as possible and see if the result takes too many bytes.