Please no Powerpoint.
I use these freeware/opensouce programs to create pictures, schemes and posters:
GIMP is "something like a Photoshop" if you don't expect anything like a Photoshop. It can do most of the things Photoshop can, but learn to work with it's interface is allegedly pain in the ass (I can't say, I only ever used GIMP and find learning to do anything in Photoshop a pain in the ass). However, if you're brave enough to learn the basic usage, you are no longed dependent on expensive software for just simple image arrangement and adding some text to them.
I use GIMP for "image manipulation" as the name says, it's a bitmap editor, it can crop, paste, add text, resize, change color levels and contrast, and blur, whatever.. delete out a band you don't like (joking) and make a TIFF, PNG, JPG, GIF.. image you need.
Inkscape is a great vector tool for creating posters that has it's baby diseases years gone. Actually, even being a vector editor, I find captioning images in Inkscape much more user griendly than in GIMP. So if you don't need to crop of modify your image, just import it as a bitmap into Inkscape and add text by the vector tools. Inkscape is renowned for it's text options, for that reasons it's particulary usefull for posters.
Inkscape's native format is .svg file (though a bit modified) but it mostly doesn't have problems exporting to PDF (with certain exceptions.. PDF doesn't support "opacity" option from Inkscape, but you can make things semitransparent by changing the alpha channel too, you just need to remeber not use opacity).
Inkscape can of course also generate bitmap images any size you want, which is great since this is often better solution for paper submitions than finding compatible vector format.
Irfanview does nothing special, but it's a very lightweight image viewer and you can easily crop, change DPI (print size), resize, change format and make basic edits to pictures that any of mentioned before.