I want to point out that my earlier figure of 192kb is not correct. There's some things before the picture as well, will return with a better estimate if someone would like it.
Also I noticed something about the spica on a swedish board called swedroid: http://www.swedroid.se/forum/showthread.php?t=19472
Basically the guy have failed flashing his phone correcly, probably because of using wrong ops file. But the interesting part is that when he tries to start the phone, instead of the normal spica bootlogo there's a message telling: "Cannot find jpg or png!"
This would indicate what I earlier have mentioned, namely that bootlogos can be both jpg and png!
There were also a guy on sdx I think that "converted" a jpg to png by renaming it. I don't know if he meant that he renamed it when he saved it (hence converting it) or if he actually only changed the file extension. Anyway it worked fine.
I honestly think that the phone can use both jpg and png as bootlogos. Jpg uses a lossy compression method and can result in smaller filesizes (nicer bootlogos!)