The only thing I'd change is to delete the word "a" before "good" in the second sentence. You might also think about spelling out the number (four) instead of using the numeral (4).
Yes, the expressions are correct and you used the correct prepositions. However, there is one small correction to make. Instead of saying "on the Pixcow website," it would be more accurate to say "to the Pixcow website." Therefore, the corrected sentence would be:
"I uploaded 4 videos about Genesis to the Pixcow website. The first video didn't seem to have good screen quality."
