Are you sure it contains the splash screens or is it just a reference to them?
Dol files are typically the binary, that being what the CPU runs, with everything else being supposed to be left to external files. That said I have innumerable examples across any number of platforms where the devs included non executable data in the binary (it is so easy and so tempting when coding something).
They are somewhat related to the ELF format which is basically the non Microsoft equivalent of PE format (which you would most likely associated with EXE).
That said if it is a hardcoded splash screen and you want to do graphics edits then I would instead use whatever tile editor you care to use (not sure what we are suggesting for the GC/Wii these days) and try to find it there.
There are any number of things from animations to tile maps that might make this edit harder but you still want to look for that.
Alternatively this may be a time you get to learn about tracing. As part of presumably the boot then in the graphics memory (which graphics viewers/texture viewers should show you) will appear the components of your splash screen.
In a debugging emulator (which can be harder for dolphin, might need to go for an older version), you would want to set a break on write to the memory area in question and then reset the game. When it loads back up with the data you want the emulator will hopefully have paused and said "this thing just wrote to it"
is a bit old but the principle is the same. If indeed the splash screen is in the DOL file it will likely have read it directly from memory, if it is external you will need to follow that back up to the DVD read (or whatever).