Welcome to the Forum and ImageJ in general.
I haven’t used DiameterJ much myself… so perhaps others here have better insight on that particular plugin - but we can at least get the ball rolling…
What are the types of images you are working with? You mentioned .gif, .tif, and .jpeg (warning: DO NOT USE JPEG for quantification analyses… this is why) - so are you converting the original images each time? Would you be able to attach the most raw/original image files you have? At least one for an example for us to test.
You can convert images to RGB in ImageJ (whatever processing you do to your images - keep it all in ImageJ if you can - to keep it more reproducible, etc) - you can search the ImageJ User Guide. Here are a few other helpful links for getting started with ImageJ/Fiji:
Hope this helps!