You can change the BIOS splash screen but it's a motherboard thing, not a Windows 7 thing, and it comes with risks, for example.. it involves reflashing your BIOS, where if you have a power blackout or something goes wrong during that process, your motherboard could become useless.
This is a common issue with all BIOS/firmware updates, whether it be a TV, modem, phone or anything else. These days it's a pretty straight forward process with most devices with a very low risk.. for example I re-flashed my graphics card bios about 10 times in 2 weeks a few years ago, to achieve lower power consumption, better peak performance and lower fan noise.. but it's still a risk.. and that needs to be said. It's not for beginners.
Some motherboards are "dualbios", meaning there is a backup BIOS for if/when the first one fails, but you'd have to look into your specific model and whether it offers that feature.
It's a lot of fuss to change the BIOS splash screen but it can be done.. it just depends on what brand the motherboard is, and what software you use for it. And how badly you want to change it.
Some brands or models are easier than others. If your computer is a Dell or some other OEM model, I'm not sure about that, you'd have to google the info about it. But ASUS, Gigabyte etc.. they are usually able to do it, with fluctuating results, for example the resolution of the picture or the ease of the process. Some are able to do a fullscreen logo, while others are limited to a set size of image. You basically need to extract the original bios from your motherboard using a utility program made by the motherboard manufacturer, for Gigabyte it's called Face Wizard for example, and then extract the image from that bios file.. then edit the image with the one you want.. then reinsert it into the bios file and then flash your motherboard with that new bios.