The output format doesn't matter as much as the settings for your project.
What are your Project Settings, as listed under the Edit menu?
Have your photos been optimized to no larger than 2000x1500 pixels (72 ppi)?
Under the program's preferences (under the Edit menu in Windows) is the option Default to Frame Size checked?
What version of Premiere Elements? What computer operating system?
Let us look at the complete situation...
Pixel Dimensions (width and height in pixels) of your pictures.
See Edit Menu/Project Settings/General and the readings for Editing Mode, Timebase, Frame Size, and Pixel Dimensions (even if the fields look grayed out).
Publish+Share/ then what (if Premiere Elements earlier than 14)?
Export & Share/ then what (if Premiere Elements 14?
As background material for "no larger than 2000 x 1500 (72 ppi)"
For the 2000 x 1500 part, please see ATR Premiere Elements Troubleshooting: PE: About Recommended Photo "Do Not Exceed" Pixel Dimensions
Any questions or need clarifications on the above, please do not hesitate to ask.