I usually save an image from google or any other website under any folder and upload them on TinyPic
by clicking Choose File. I would resize the image between 320x240 or 640x480 then click upload now. You might need to verify so that you're not a bot. Once you get through after clicking upload now you will see 'Share This Image', copy the link under Direct Link for Layouts. Now you may upload an image on SF with your post. Click insert image and paste the link you copied and click preview. If you like the size then you may continue. However, you can change the image size. Here's an example what the image size looks like:
Hope this answers your question.