If all you want to do is scale the image, as you say, there is a simple way to do this. Go into the material browser and click on the material in question. In the right hand pane, click on the Appearance tab. This will show what image file is associated with the material. Click on the little down arrow next to the image which will display a list of choices - pick the bottom one "edit image". Within the dialogue box that appears, the image file is shown with a vertical and horizontal dimension showing the scale of the image as it will be used in your model. You can change the x and y scale of the image to be whatever you want.
Another useful capability is that on the Appearance tab, instead of clicking on the little down arrow, you can click on the image file name. This will pop up an entire library of image files to choose from which you can associate with the material instead of the default image file. I regularly use this to change the color of a material. I simply edit the image file in Microsoft Office's photo editor which has a pretty simple ability to change the color and brightness/contrast of an image file. Save it to a new name because the files in the Autodesk folder are write-protected. Then select it as the image file to be associated with your material and, voila, you have created your own material (green siding instead of tan siding, or orange brick instead or red brick).
You can also use your own image files, or third party image files, but you have to be careful that they are set up to tile correctly. I have a whole library of different stone images from Cultured Stone's free StoneCAD software which are intended for this sort of use and therefore do tile properly. This multiplies many-fold the range of stone looks you can achieve in Revit.
