Finding Symmetries in Squares
Python 2, 460 452 437 bytes

exec"""def f(L):
 if[]==L:
  X{2}[map(" ".__lt__,q)for q in G]);Z{2}zip(*X));C=Z==Z[::-1]or X==X[::-1]
  if C:print"\\n".join(map("".join,G))
  return C
 x=L[-1];T=S-x+1;R=range(x)
 for n in range(T*T):
  i=n%T;j=n/T
  if all({1}=" "{0}):
{0}:{1}chr(32+len(L))
   r=f(L[:-1])
{0}:{1}" "
   if r:return r""".format("   for a,b in[(a,b)for a in R for b in R]","G[i+a][j+b]=","=filter(sum,")
L=input()
S=sum(L)
G=[S*[" "]for _ in[0]*S]
f(L)


Only mildly golfed for now, but here's something to get things started. I tried using exec for lines 10 and 12, but for some reason it didn't let me.

Input the list L via STDIN, e.g. [2, 1, 2, 2, 2]. The program simply tries every possibility of placing squares in a sum(L) x sum(L) grid.

Sample output (blank lines removed for compactness):

[2, 1, 2, 2, 2]

%%       
%%       
$$       
$$       
"        
##       
##       
!!       
!!      

[2, 4]

""""  
""""  
""""  
""""  
 !!   
 !!   

[2, 1, 1, 1]

$!!  
#!!  
 "   

[1, 1, 2, 3, 5]

%%%%%       
%%%%%       
%%%%%       
%%%%%       
%%%%%       
$$$##       
$$$##       
$$$"!       

[1, 4, 1, 8]

$$$$$$$$      
$$$$$$$$      
$$$$$$$$      
$$$$$$$$      
$$$$$$$$      
$$$$$$$$      
$$$$$$$$      
$$$$$$$$      
# """" !      
  """"        
  """"        
  """"        

[8, 1, 4, 1]

$   !!!!!!!!  
    !!!!!!!!  
####!!!!!!!!  
####!!!!!!!!  
####!!!!!!!!  
####!!!!!!!!  
    !!!!!!!!  
"   !!!!!!!!  

(The algorithm starts placing from the last square first, prioritising left then up)




Slightly less confusing version (452 bytes):

def f(L):
 if[]==L:
  X=filter(sum,[map(" ".__lt__,q)for q in G]);Z=filter(sum,zip(*X));C=Z==Z[::-1]or X==X[::-1]
  if C:print"\n".join(map("".join,G))
  return C
 x=L[-1];T=S-x+1;R=range(x);V=[(a,b)for a in R for b in R]
 for n in range(T*T):
  i=n%T;j=n/T
  if all(G[i+a][j+b]&lt;"!"for a,b in V):
   for a,b in V:G[i+a][j+b]=chr(32+len(L))
   r=f(L[:-1])
   for a,b in V:G[i+a][j+b]=" "
   if r:return r
L=input()
S=sum(L)
G=[S*[" "]for _ in[0]*S]
f(L)