This is LittleJoe-42, with a little knowledge about inserting your own .PNGs into GTA4.
First, you need SparkIV. This is a must! .PNG sizes are based on a binary system. All will have dimensions of 128, 256, 1024. I used Paint.NET to create my billboards. First, I exported the .PNG from a GTA4 .wtd file. Next, I open it into Paint.NET, which will automatically set it to the proper dimensions (if you're one of those people that likes HD textures, "resize" it to double dimensions). Create a new file, make your billboard (I take images from Google Images myself), and put it on a layer above the original billboard, adjusting for size and proportions. Export the final .PNG (DON'T ERASE OVER YOU'RE ORIGNAL, IT'S THE BACKUP), and reinstall into the .wtd using the "edit" function.
Finally, is you're changing large textures, there's most likely an associated "length of distance" texture. This texture, for example, as in billboards, is half the size of the original billboard. You can find these files by entering "lod" into SparkIV's .IMG search engine.
If you're really serious about making your own billboards, I suggest looking at my own billboard mods, or ZBNYNCs'. In fact, if there's any place you want to make a change to, most likely someone has done it before you. Simply look up someone else's, and use the same texture locations to make your own.