"ICO" is an image format used by Windows to store program icons. Each ".ico" file contains the raw image data for the icon plus an image mask containing transparency data for the icon. When dealing with a large number of icon files, some programs store one or more ".ico" files in a ".dll" file, or Dynamic Link Library. Windows uses ".dll" files to store crucial system files and other information that is shared between various programs and external components.
Sign up for a free trial of GConvert 5, a program designed to edit and manage icon files. GConvert can store or convert one or more ".ico" files to a ".dll" file. Click and drag your ".ico" files to the main GConvert window. Click "Library" and select "Compile Library to a DLL file (32-bit)" to start the conversion.
Download and install Axialis IconWorkshop, an icon library management tool available as a free trial. Create a new icon library by clicking "File" and choosing "New" and "Library" from the menu. Click and drag the ".ico" file or files you want to convert into the IconWorkshop window. Select your files in the window. Click "File" and "Export" to start the conversion. Choose "As ICL or DLL" and enter your file name.
Install IcoFX, a free icon library manager. Click "Tools" and select "Resource Editor." Click the "Add Icon" button in the toolbar and select your ".ico" files. Click "Save" and enter a file name. Enter ".dll" at the end of the file name.