Those popups occur in many places on the web. If you ever receive a notification about spyware, malware, viruses, etc. and the top of the popup window does not have the name of your protection software, then don't click it - it's a phish. Similarly, ignore 'Active-X' or other plug-in notifications if the pop-up window does not have the name of your web browser. To be really safe, don't Ok anything if you are not sure of the source and what it will do.
Now, as for image hosting. Free is nice, but web hosting costs so little. US Website Design and Hosting will give you 100mb of space for $50 a year. Even if you don't want to have a web site, you can still use it to host pictures. Or you can do both; my site has a directory called /images which contains pictures not directly part of my web pages. Many hosting companies will even set up your site as a wiki as part of the package, which means it's easy to upload pictures. USWDH does but I opted to use MediaWiki because it's used at my University and I know how to 'tweak' it a bit. I'm not affiliated with USWDH; there are a lot of good hosting companies out there. But I started my own website many year ago when I was doing some consulting and have maintained the site ever since; prices have only dropped and you get features today you wouldn't have dreamed of back then. I'm paying a little over $100/year for 3GB (that's 30 times the $50 level) because I sometimes host some large files for classes at NAU as well as my personal site and photos. That's about $8 a month - I don't miss it.