You can upload images directly to App_Themes for use in a stylesheet via the Theme tab in Site Manager -> Development -> CSS stylesheets -> <your stylesheet>. However, this will place the files in the same location as your project folder under the directory ~/App_Themes/<your stylesheet>. If you want to change this default location, you will need to modify the following file ~/CMSModules/CssStyleSheets/Pages/CssStylesheet_Theme.aspx.cs around line 27.
Applies to: Kentico CMS 6.x
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