I noticed that the PNG images used in Open PS2 Loader can be made a bit smaller using the PNG optimisation program PNGOUT.
For example, I tried running PNGOUT on the few largest PNG files. Here are the before and after file sizes:
app.png: 6042 -> 5828
config.png 5174 -> 5174 (no improvement)
eth.png 5373 -> 5211
exit.png 4236 -> 4040
hdd.png 7241 -> 6990
logo.png 63906 -> 55319
save.png 2978 -> 2759
usb.png 4905 -> 4803
Because most files are fairly small the optimised versions are only a couple of hundred bytes shorter. But the optimised logo.png is about 8.4KB shorter. The total saving for those PNG files is 9731 bytes. Not a huge difference, but it would speed up loading the OPL ELF file ever so slightly.
I ran all the PNG files through PNGOUT and the result is attached. Total bytes saved: 12897
Update: I tried again with different settings and got the total size down to 105877 bytes (vs 118956 for the originals), saving 13079 bytes.
Update 2: Total size down to 105642 bytes, saving almost exactly 13KB.
Update 3: "Fixed" optimised PNGs zip attached. It turns out OPL can't handle paletted PNGs with transparent colour. It needs all the images to be RGB+alpha.