Are your images in a sequence? You can use gprename or a similar program to rename your files. The easiest way for ffmpeg is to just use numbers and then use the appropriate "%d":
Here's a quick script that will rename all images into a sequence like 0001.jpg. I'm not 100% sure it will do it in proper order, so backup first. It will also not work on files with spaces in the file name and some other characters:
00n.jpg = %03d.jpg
000n.jpg = %04d.jpg
Also make sure there are no missing images from the sequence. For example, if 0018.jpg is missing, then it will stop encoding at that missing frame.
ls *.jpg | nl -nrz -w4 | while read a b; do mv "$b" $a.jpg; done
You can control the frame rate (-r) and bitrate (-b) easily. -f image2 might be unneeded. You also don't need to dump your final movie to /tmp:
ffmpeg -r 15 -b 800k -i %04d.jpg a.mpg