Welcome to Premiere and to the forums. Unfortunately, you posted this question under OnLocation, which is a separate program that ships with Premiere Pro. While you might get an answer here, your odds will be better if you re-post in the main Premiere Pro forum.
Try setting your text by creating a Title with the built in Titler.
You problem maybe something to do with the png file ( size /quality)
Along the lines of Craig's suggestion, what is your Export/output format? The reason that I ask that is that with some, like DVD, you might actually want to soften the Text just a bit, for DVD to TV display. If FLV, or most others, you will not.
I also agree that Titler is a great utility.
If that is still causing issues, you might want to give us details on your Sequence Preset and full details of the Export/output.
PNG is a bit map format. Did you rescale the PNG image when you imported it to Premiere? Try scaling the PNG image to the size you need for your time line. If you are working in HDV, for example,rescale the mage in photoshop to 1440 x 1080, PR=1.33 before you add the text. Depending on your preference settings, Premiere will rescale an imported image to fit the sequence dimensions. You generally do not want to let this happpen. This can mess up text resolution.
Best approach is not to add the text to the image, but rather, use a transparent overlay with a vector-based titler.
Is your program monitor set for full resolution? I noticed it defaults to a lower value.