Dropbox ko ni kaam ta garnu parne jasto laagcha, the best method afaik to post images on steemit is to right click the image you want to use from the webpage > open image in new tab > copy the url from the address bar > paste the link at the text box to submit a story. This works most of the time. If you're still having trouble then uploading the image to a 3rd party image storage site like imgsafe.org, steemimg.com and copying the url from there might help.
I use https://steemimg.com to upload images for steemit, you can copy the url for Markdown from the embed list there and paste it over here. It was created by a steemit user.