There is no generic way to transfer files. You need to decide what app you want them in and transfer them to that app. Each app has different ways, often more than one, of having files transferred to it. These can include email, File Sharing through iTunes, your computer's browser, or the perennial favorite DropBox.
If you just want your photos synced to the iPad for viewing, you can sync folders from your PC through iTunes:
iPhone, iPad and iPod touch: Syncing photos via iTunes Most people prefer to create a folder and subfolder specifically for this, and "copy" the photos you want to sync. This way you don't have to worry about messing up the originals if you decide to re-arrainge the folder and files later.
Break things down by what files you want on the iPad, and what you want to do with them. Find the apps you like best for each task and the howt-to-transfer issue will answer itself.
A bit of background. Apps in iOS live in a sandbox. With very few exceptions apps can not directly access files from other apps. Instead you use the Open In option in the originating app to copy a file to any destination app that has registered with iOS it's ability to handle that file type. This means there is no common file space where you can dump files for access later, and why there isn't a simple one solution for your question.
It sounds like a pain, but once you get used to it the system works fairly well. It's another approach to the problem that makes apps central to the system instead of files.