Different manufactures use different device drivers to do the same basic thing. Motorola for the most part attaches your device as a USB Mass Storage device and copies files at a much faster rate then Samsung does... and I believe HTC does the same thing as Motorola. It all in how THEY handle it, and nothing to do with how your PC is working.
In the case of your png files. I have to believe it's something you have setup on your PC that is causing you to see that message. I have never seen any message like that on any of my PC's... ever!!!
Can't say it's something "screwy" with my PC's ... on both my home pc (older laptop with Win Vista) and on my work PC (relatively new workstation with Win7) the same thing happens ... and I don't have any odd programs or settings on either. I did some searching on the web and it appears this is a Samsung issue with how they have the phone setup to identify itself to a PC. When you plug it in, it notifies you with the following message: "Connected as a media device". If you tap that message in the notification panel, it takes you to a USB Settings dialog where you can choose to have your phone connect as either a "Media Device" (MTP) or a Camera (PTP). However, as said before, there is no option to connect as "USB Mass Storage".
And if I understood what I read about this issue, both the MTP and PTP settings make the computer think that the device is only capable of "using" certain file types ... and thus warns you that you may be copying a file type to the device that it's incapable of using/viewing/processing.
I also found, that if you root your phone, there are a couple of apps that will allow your phone to emulate "USB Mass Storage" when connected to a PC ... so that you can avoid these messages and get faster file transfers.
Again ... I am not a true techie, and can only assume there's a reason for Samsung's decision to make the phone only connect in either MTP or PTP mode ... but seeing as how the phone can handle most any file, it seems odd to me that it's setup this way.
I may email Samsung to see if they will respond on this issue. I won't hold my breath though.