Go into your phone's camera settings and choose a smaller picture pixel file size, if available.
I use a Samsung Galaxy Express 3 (AT&T) and use the 1280x720 pixel size pictures, which post just fine. I have two other sizes to choose from, which are 640x480 and 320x240 but have never tried using them. I seen some discussions on here about people's complaints that the "picture/file is too big" and the only thing I remember as a fix is to make that camera setting change & downsize the pic/file. If I hear of any other possible fixes, I'll post them here.