I don't care how much memory an image takes - that's a behind-the-scenes thing of no relevance. What I do care about is how long I have to wait for it to appear, and whether it is in-your-face because the posted thinks it must dominate my view, or is illustrative and enlargeable if I wish to see it better.
As noted above, plonking the full thing in the text fails both of those. Specifying some 'sensible' size also fails since you don't know the size of my browser window. Just use the thumbnail and if I need to see it better I can click on it, otherwise it's nicely out of the way and not detracting from the text.
There are exceptions, but most inline images don't qualify.