ok im opening pandoras box here.(treatment vs framed wall art)

I have four framed glass enclosed water colors in the room and have no problem.  I have room treatments, panels, bass traps and tube traps in the room as well.  My system is full range down to about 25hz with the speakers and two subs.

When the speakers were spiked to the carpet over concrete floor I used to have a strong resonance at around 125-250hz that would vibrate the walls if played loud and with music that had a lot of content at that frequency.  Removing the spikes and replacing with Stillpoints eliminated that problem completely.  The Stillpoints do not puncture the carpet or pad.

I would suggest decoupling the speakers from the floor.  Townshend pods or podiums would be excellent if funds permit.  

Brightness is a condition that exists in a system when there is excessive treble energy relative to the rest of the audio spectrum ... it is an out of balanced condition that can be caused by either your equipment or generated from your room 

Digital components with high amounts of jitter can produce this bright condition ... RF sneaking into the signal path can cause brightness ... even uncontrolled vibration can raise treble energy  

Poorly designed components that don't produce adequate amounts of bass can also cause the treble to stick or stand out and present a bright or tipped up condition  

This bright condition can also be caused by your room when conditions cause extended modal ringing in the high frequencies ... here you have extra long RT60 times were the note just lingers more than it should  

The two most common perpetrators in the room are ... Echo Slap were the sound energy just keeps bouncing and lingering to long  between two parallel walls ...  and the wall/ceiling interface where the wall meets the ceiling ... here energy traveling up the wall combines with energy traveling along the ceiling in what is known as the laminar effect combine at the wall ceiling interface and are reflected back into the room ... unfortunately the combined energy that is reflected back into the room has much more energy than the original note  and causes the note to linger or ring well beyond it's normal or natural decay time  

If there is any problems created by your glass framed pictures it's a would be caused if your pictures are hung at the point where the direct wave above the room's transition point coming from the speaker strikes the side wall  

The reflection off the glass would be reflected at a different rate than if the wave struck the sheet rock wall ... now the problem is of different arrival times at the listening position of the direct wave from the speaker arriving ahead of the reflected wave off the wall ... the glass may reflect more energy than the sheetrock would have  

Your ear would perceive this as two separate notes because the same note arrives at different time intervals ... this would blur the imaging .. mask low level detail and effect the sound stage but I doubt very much that it would tip or increase treble energy 

As Geoff has suggested try removing the pictures and I'll bet your room still sounds bright as I think the brightness is being caused by some other condition  

One more point of interest is even if you remove the pictures from the room ... the room will still respond and vibrate in sympathy to it's primary resonate frequencies based on the rooms dimension  

The problem here is every primary note has harmonics and the harmonics from this low frequency distortion can easily reach up into the midrange and lower treble coloring both 

Success in audio will only come when you realize that the room and not you is in control ... so you need to take control of your room's anomaly