How do I frame odd-sized pictures?

I had pictures smaller then my frames but had mattes. So cut them to size and the mattes are just like a border within the frame. Looks more finished too.

go to thrift stores, they sell empty frames and pictures in frames. You can remove the old pix to use. If you need one bigger , cut the sides or top in the middle and glue a piece between the 2 pieces to add the size you need. hope that makes sense. Even if the frame is ornate, a straight piece added in the middle of the side of top to lengthen or widen works, you can even add embellishments like the little wooden pieces they sell in the craft stores. If you find some with broken glass, you can use plexiglass which is cheaper than glass and can be cut to fit. I have even just used without glass because most art does not have glass over them.

make your own mats here are how to videos https://www.youtube.com/results?search_query=DIY+making+cutting+mats+for+pictures

You might look online at "Art to Frames". They are the least expensive place I have found for custom framing, about $30 for a 16" x 16" custom frame including glass and hanging hardware.