Reminding all that PJ previews over 10k do not show in the front page when first added and will not show in the weely showcase (WS) or Hall of Fame (HOF)...
You can easily check your previews with Yanrishatum's amazing Image Specs Tool.
Note that .gif files almost always are ok but if you output .png files then .png8 is always better than .png24
If your file is big in size then its likely you are using .png24
A lot of issues arise from gamma corrections and truecolor (png24) vs. indexed color (png8) and some is explained HERE
Your pixel app already likely has options to control your file sizes but if you need an online editor that works really well then try THIS one. GifMaker.me
THIS article on optimizing gif files explains some details.
THIS article on general page weights has some useful info too.
THIS article in on compression and also has some relevant info.
These links have been added to the links section for future use.
Thanks 8 Bit Dreams