First, your tiles are extremely pillowshaded. This is a common mistake for beginner artists. What it means is that you essentially don't have a lightsource, everything just gets lighter inwards, making it look like pillows.
Second, you only really have one tile, but with different walls on the edges. Having one tile would be fine if that tile really worked, but your pattern is much too erratic to look at pleasantly, it also shows the grid way too well.
The sides are very small, but the big problem with them is that they are almost unnoticeable. The background is grey and black, and the sides are also grey, with a different level of contrast than the background.
I don't really know what I'm looking at. Is it a floor, or a wall? I'm assuming a floor.
I'd suggest taking a few steps back, deciding what kind of floor you want to do and then do some research. I would also recommend doing just the floor tiles. If this is supposed to be able to be used by a lot of people, having the walls there is unnecessary. Better to do some walls, and add them to the zip file in a separate image file which the user could then place on top of the tiles inside Construct.
I'm also sensing photoshop here.
A little something on shading, although it's aimed at pixel artists.
http://www.natomic.com/hosted/marks/mpat/shading.html