Depends on the content of your meme. You can post everywhere as long as it meets the flair of the stream and doesn't violate the stream posting rules. Check the stream descriptions.
e.g. Durch soccer fans are called 'Oranjes' so you may submit a picture of a dutch soccer fan to the 'foreign language'-stream or 'international'-stream.
If by 'orange man' you hint at President Trump intending to either root for POTUS or make some decent joke of him you're to post in 'politics' (where the Theme Week originated). Comment sections are not velvet gloved there though.
To explicitely mock President Trump in a nasty meme 'politicsTOO' would be a better safe space to go with.
If it's an unpolitical punny meme just give it a try in the fun stream or some funny meme stream.
This is how to add tags:
go to image page and click 'edit image' before clicking 'submit image to imgflip community'
Or else in the new submission window I think you can also add tags&title last-minute.
u forgot to tag it though. make sure to apply some of those little grey tags as eg. given here. imgflip.com/i/3zvm40
only then your image will show up when people click on those tags to see the theme week submissions.
Caption this Meme 
