No a GUID is actually a good idea to avoid naming collisions. Make sure to avoid spaces.
For relative paths always use ./ in front of the path if you want to start from the current folder. I´ve had issues where I didn´t do this.
so in your case:
<asp:image id="Image1" runat="server" imageurl="./Ads_Images/7f0e7ef3-4cb8-40e0-a835-00314768a70a70-515.jpg" xmlns:asp="#unknown" />
<asp:image id="Image2" runat="server" imageurl="./Images/imagenotavailable.jpg" xmlns:asp="#unknown" />
Secondly, is Ads_Images a folder in the same level as the Images folder?
Thirdly, if the user saves the image to this folder, the control needs to be updated in order for the image to show.
In your case you need to wait until the images exists on disk and or reload the page or call some ajax update script to update the image.
Hope this gives you some ideas.