Sure- just plug it in via USB.
You can have them import into Photos, or use Image Capture to download them to where your want.
I’m thinking this is a bug in Photos. It looks like copying pictures via Command-C or the Edit menu will paste them as lower res pictures and the resulting file is appended with the “thumb_” prefix. So your photos in the Photos app are probably still full-res, it’s just that invoking the “Copy” command seems to paste them as thumbnails. Instead of invoking the “Copy” command, try dragging a picture from the Photos window into the Finder window. For me, that creates a full-sized, correctly named pic. Is there some other way you think the pics are really thumbs, other than what happens after you copied them? Like, if you view them from in Photos, do they look grainier than they should? Because the fact that you say doing a get info (from inside Photos, I presume) shows them to be full-sized 2-4 MB sized pics tells me your photos are fine.
As for where Apple is actually storing your damn photos, it can be wherever you want. If you drag a photo from “untitled folder 8” into Photos, the original is stored in “untitled folder 8.” There are some serious drawbacks to that (notably, don’t ever move the pic or the folder it’s in because Photos will lose track of it), but you can do it if you want. Just go to Photos preferences and uncheck the “Copy to the Photos library” box. I really don’t recommend that.
As for your crashing apps, have you updated them to Yosemite-compatible versions? It’s an easy thing to forget.
See post 10.