I'd guess that the senders are changing the size of the picture to small (100 or 200kb), others are selecting a larger size. Probably the way that they have their mms settings set up and not your phone causing the issue since you receive some pics at a viewable size.
The default messenger app doesn't appear to have an mms sizing option, but if you were to use GO SMS, you can choose to preserve the image size and allow up to 5MB mms size. I'm not sure if that will help when receiving a small file of 100kb or so, but it might be worth a try, plus you'll probably like the GO SMS a lot better than the default (if that is what you are using).