If I remember right, a year or two ago there was a discussion about resizing photos. I had been doing it one-at-a-time with picresize.com but someone in our group had a way to select several photos and resize them in a "batch". You could select the ones you wanted to resize then drag them over into the program and they would resize a batch of them all at once. The resized photos came back into the same album but in a sub-folder of "resized" photos. I found one that put them next to the original but I liked it in a separate sub-folder. Does anyone remember the name of that program?
Picresize.com has a batch feature, just select "multiple pics".
As you had suggested to me a while back, this is the same site I have been using, but found the 'batch' re-sizing to have been less usable. (Harder to 'guess' the size!) I've just re-sized the singles, downloaded to 'pictures' on my computer, then upload as wanted to MTFCA. Delete them from my computer as appropriate. A simple approach for my simple mind? (It works!)
irfanview is a free tool that can do batch resizing, along with a whole bunch of other image processing stuff. You can do things like take a whole bunch of pictures and resize them all so the longest side is xxx pixels.