Using the Highlight blue skin, I cannot change the logo in the upper left corner. I followed the directions to open the logo.psd file, edited it in the exact same dimensions, etc. There is no .jpg file to replace in my images folder.
There is however a logo.gif and logo.png file under images/styles/highblueblue/style/.
So I did a little searching and saw you advised to replace the .gif. I saved the file as a .gif with the following directions...
"To save your logo with .gif go to File > Save for Web & Devices > Gif 128 Dithered and Change Colors to 256 > Save".
This made no difference. If I replace the .gif, nothing changes at all. If I replace the .png file, the logo is gone entirely but there is a broken link box in its place.