Hi Didier,
I'm going to respond to your questions in order of your posts.
1) There's actually two ways you can upload the image. You can upload the .PNG to /wp-content/themes/flashlight/images/ using FTP or you can upload the .PNG using Media > Add New. Whichever way you choose you would note the image path and change your CSS accordingly. For example, if you uploaded the image via FTP you would add this to your custom.css or Quick CSS.
.entry-mini {
background:transparent url(/wp-content/themes/flashlight/images/background-transparent.png) repeat top left;
}
As far as the name of the image you need to replace, there is no image you need to replace.
2) You have two options for storing customizations. The first is adding your customizations in custom.css located in your /wp-content/themes/flashlight/css/ folder. The second is the Quick CSS box under Flashlight > Theme Options > Styling. Really the choice is yours as far as which to use. I would suggest though that if you're playing around use the Quick CSS then once your customizations are the way you want them copy everything in that box into the custom.css just so you'll have a record.
3) The box will automatically resize according to your text. If however you find you want more space around the edges I suggest adjusting the padding since defining a specific height may present some positioning issues.
.entry-mini .box { padding: 30px 30px 30px 30px; }
The four 30px represent in order - top spacing, right spacing, bottom spacing and left spacing. So you actually do something like 60px 30px 60px 30px which would give the box 60px of space on top and bottom, 30px of space on right and left.
Hope this helps!
Regards,
Mya