You should not use your web.config file to store these file paths. Web.config as it's name suggests is for your web applications configeration settings not storing data.
That however is not the problem. If all uploaded images are stored in the same folder then you know the location so you only need to store the image name.
If the images where in a folder called photos that was in the website root directory the url would be.
<img src='/photos/imagename' />
You can bind all images in a folder to a grid like this (note the path is /photos/ in code and grid imagefield)
Code:
Dim dr As New DirectoryInfo(Server.MapPath("/photos/"))
gridView1.DataSource = dr.GetFiles
gridView1.DataBind()
Code:
<asp:GridView ID="GridView1" runat="server" EnableModelValidation="True">
<Columns>
<asp:ImageField DataImageUrlField="name" ControlStyle-Width="100" DataImageUrlFormatString="/photos/{0}">
</asp:ImageField>
</Columns>
</asp:GridView>