I tried dropbox and hated it.
I just opened a flickr account, and while not straightforward on ios, it is easy peasy on a desktop. Plus, they automatically resize it for you too.
So, step by step for flickr:
1) Navigate to the photo you want to post and click it to make it full screen
2) Click on the 'share' button, which is on the lower right hand side, and looks like an arrow pointing to the right
3) Select BBCode, select the size of the picture you want (medium is good), and copy the code it generates for you
4) Paste that code in your message. That's it. You now have an image.
By default, flickr will add some extra information, like the picture title and author, and will also make your image a link to the flickr page. If all you want is the photo and nothing else, only keep the part of the code which starts with {img} and finishes with {/img}. (they are actually square brackets, not curly, but I can't put that in without it thinking I'm trying to post an image)
The reason that it isn't straight forward on ios is because the flickr app doesn't have a share button. Hopefully that add that feature in the future.
Thankfully, flickr is integrated into ios. So my routine for posting photos I take is to take the photo on phone, then share to flickr (a feature on ios, it even lets you select an album to put it in, like SOTD
, then open flickr on my desktop and follow the steps above.