I use imgur now. I wouldn't recommend postimage. I did use it for a long time then all the links changed which lost me all my photos on numerous forums. I stuck with it for a while, then it happened again - so went with imgur. The nice thing with imgur are the modifiers by adding a letter at the end:
Here is the list of all the modifiers for dimensions (not all are always available)
s = Small Square (90×90)
b = Big Square (160×160)
t = Small Thumbnail (160×160)
m = Medium Thumbnail (320×320)
l = Large Thumbnail (640×640)
h = Huge Thumbnail (1024×1024)
Like this e.g. for huge
https://i.stack.imgur.com/nnnnnn.jpgbecomes
https://i.stack.imgur.com/nnnnnnh.jpg