Sunday is intentionally left empty because the "This Week" post doesn't go up until late on Saturday if I get things right, and (hopefully) early on Sunday if I don't. So if we want to give it its day in the front, Sunday is the day for that to happen.
In theory, Sunday is the one day that you can't have.
In practice we're not exactly overflowing with main posts so even though we could have a post in addition to the Board Business post on Thursday or the Decontruction roundup post that I swear I'm going to try very hard to get done by this Friday, there's no reason we'd really want to double up things.
Which means you're more looking at Monday, Tuesday, Wednesday, Saturday (with the understanding that a Saturday post should go live early to minimize the weekend post taking away from it.)
I'd recommend cute animal Tuesday, but that's just me.
On an entirely unrelated note, does anyone have anything ready for Monday?
I think Ana knows the answer to that. For my part all I can say is:
1) It depends.
2) It's better to host the image yourself than have it load via link because then you're not stealing their bandwidth.
Part "1)" is where the true answer to your question resides, and I think Ana knows what it depends on where I do not.
What I do is this: I host the image myself, credit the owner, provide a link back, and then delete the image from my site and files after a month or two has past. Whether or not that falls under fair use is controversial.
There are a number of free image sites, though, where all you have to do is provide credit. I can't remember the site names off the top of my head, but I used cute animals from there in.... March? when I did a week of open threads when my dad got sick.
So if someone was to dig up those links, you'd see which site I used.