Validation for FileUpload and DropDownList one control must have value using jQuery in ASP.Net

Hello Guys ..

I have a requirement like User Will Upload a image file using file upload control ,and in the same way he can select the images from the existing  drop down there is a checkbox when user selects checkbox existing images will be populated in drop down either he can select a checkbox or upload a image through file upload, now how do i keep the validation to upload a image based on two combinations..

Help..??