I have had the same problem before. In my case the modid was not correct. For example, if the folder structure in the mod .zip file is:
L <folder> testcraft
L <folder> mod
L mod_TestCraft.class files
The modid in the mcmod.info file must be testcraft. I was using the wrong text for the modid so I also had no errors but the text and .png file for my mod was not showing up.
It seems to me that the modid is not necessarily the name of your base mod .class file, but it is the exact text used to name the folder in your .zip file. At least that is what fixed the problem for me. I hope it helps you as well. Good luck!