This is likely due to the size of your photos (megapixels).
Most newer digital cameras take a photo at 4-10 megapixels.
At 4 megapixels a camera will take a photo that is approx 1.5 MB and 2270 x 1700.
What you really want to do is reduce the photo to roughly 140 KB and 800 x 500 for the gallery and more like 80 KB and 600 x 300 to show a photo on a thread. Most digital cameras come with programs that will allow you to reduce the photos one at a time or as a "batch" and do as many as you want at one time.
Snow and Mud has a limit to the size of photo that can be stored in the gallery. The programming will reduce the size somewhat but if the photo is too big then the system chokes and won't upload.
Check you programs that came with your camera, chances are really good that what you need is there.