It's easiest to resize before you upload, Microsoft Paint is easy.
Right click your picture
select Open with Paint
select resize { upper left corner} choose resize by pixels make shure "maintain aspect ratio" is checked
then play around a bit to find something you can live with.
My Nikon produces files in the 6kX4K range that are about 9Mb each
for uploads I knock that down to a 800 x 600 pixel range which works pretty well
when finished, save the file, then use it for your upload.
Upload them to photobucket then copy link and paste to your post, this makes it easy to go straight from your phone to thread, it's same way on TG. Then you don't have to resize anything
Have you noticed that some pictures that you post from your phone are larger than others? I've taken multiple pictures with my phone and then posted them up on the same thread and for whatever reason some come out larger than others.