Our bright white photo paper may be thicker thank your printer is used to, so you may need to either help push it through gently with your hand, or use the flat feeder (if your printer has one). If it still won't feed, try using your fingers to slightly bend the first 1/4 inch of the sheet up, and cut off the top left corner. That should help feed heavy papers through.
To feed the smaller 4 x 6" paper, use the arm for envelope feeds, pushing it over to the correct width, and the paper should feed through. If you have an older printer, you may need to tape the smaller paper onto a sheet of 8-1/2 x 11" regular printing paper. Line up the small sheet in the top left corner of the big sheet, and put a piece of scotch tape only along the top edge. This can trick your printer into thinking you're sending through a sheet size it prefers.
Please check the online user guide provided by the manufacturer who made your printer for more details.