See below a simple implementation of IImageSource using MemoryStream which can be used directly from FlexiCaptureProcessor via SetCustomImageSource: 

// IReadStream interface is the interface for a read stream. This interface and all its methods are implemented on the client side. A read stream may be implemented as reading from file. 
class ImageSourceFromStream : IImageSource, IFileAdapter, IReadStream{
    public ImageSourceFromStream( Stream _stream ) {
        stream = _stream;
    }

    // IImageSource  
    public string GetName() {
        return "Stream"; 
    }

    public IFileAdapter GetNextImageFile()  {
        if( stream != null ) {      
            return this;    
        }  
        return null;
    } 

    public IImage GetNextImage() { 
        return null; 
    }    

    // IFileAdapter  
    public FileMarshallingTypeEnum GetMarshallingType() { 
        return FileMarshallingTypeEnum.FMT_MarshalByValue; 
    } 

    public string GetFileReference() { 
         return ""; 
    } 

    public IReadStream GetFileStream() { 
        return this; 
    }

    // IReadStream  public int Read( out byte[] data, int count ) { 
        data = new byte[count];   
        int len = stream.Read( data, 0, count );   
        return len; 
    } 

    public void Close() {   
        stream = null;   
    }
};