As of ODK Collect v1.1.10 (currently in beta), it is possible to specify a maximum size for the long edge of an image at the form level. This means different images within the same form can have different sizes as desired. This is not yet implemented in XLSForm but hopefully will be soon (https://github.com/XLSForm/pyxform/issues/137).
If you'd like to use this feature before XLSForm supports it, you can generate your form as you normally would and then edit the XML to add the size constraint.
Here is a sample form with 4 image prompts: one that's unscaled and three scaled down to long edges of 1024, 640 or 100: max-pixels.xml (1.3 KB)