I apparently actually did two of these on different occasions, using different, restricted Unicode character ranges (ones that only look at the value of the character as a whole, no subpixel rendering). Can't find the (newer) color one, but the black-and-white one:

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 ░▒▓▓▓▒▒▒░░░░░                                                                
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        ░▒▓▓▓▓▓▓▓▓▒▒▒▒▒▒▓▓▒▒▒▒░░░░░░  ░░░░░▒▒▒▒▒░░▒▒▒▒▒▒▒▒▒▒▒░                
         ░▒▓▓▓▓▓▓▓▓▓▒▒▒▒▓▓▒▒░░░░░░      ░░░░▒▒▒▒░░▒▒▒▒▓▓▓▓▓▒▒░                
           ░▒▓▓▓▓▓▓▓▒▒▓▓▓▒░░  ░░░░░▒▒▒░░   ░░▒▒░░░▒▓▓▓▓▓▒▒▒▒░                 
            ░▒▒▒▓▓▓▓▓▓▓▓▒░░   ░░▒▓▓▓▓▓▓▒░░  ░░▒▒▒▓▓▓▓▓▒▒▒▒░                   
                 ░▒▒▓▓▓▓▒░   ░▓▓█▓▓▓▒▒▓▓▓▒░  ░▒▒▓▓▓▓▒▒▒▒░                     
                    ▒▒▓▒▒▒░░░▒▓███████▓██▓░   ░▒▓▓▓▒░░                        
                      ▒▒▒▒▒▒▒▒▓████▓▓████▓░   ░▒▒▒░                           
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Is generated by the program with this uuencoded source:

begin 644 unicode_image.tar.xz
M_3=Z6%H```3FUK1&`@`A`18```!T+^6CX"?_!IM=`":8269IV=F-Y,!%M1>4
MZX(9LR,YMG1:D2XCM%DZ,0N4%>'\;I?0D"7/H:OI15<M6G@8HO/[&((J'=B.
MXY\G/[7D"Y)B$O)IC]DM9Y@^\4T?'9(.Z.4+7IDF/T0&\7`M5+G#=C!?(>(U
M+-C2%PZ!"(Q'Z_/D^%"[PVKX:A:OKH5WF?AQ=CD_AAS]<3<THTMC0S8FG\<A
MZ;A-_9H?)5S'YG5A?0WUQ7FR0+IS\0AEUYY9QFMY?"$);\U%_R0NK(ZZ/Y&J
MVA;@#O-P.6W8PW']0U"<S'NHB=.(/)OX[<1&UF@M8+GXPGVFQB_+K/WD01ZO
MO#+E!CK;^`V-WGH^?0V5M!IK[KR&]`IR<>6D+ONPJT6E\CZJ^KKZ,W?3O"2K
M!/GHQ&TDN';;P#UC;)+HRPH$`_8JM#ZV`\I6=,PO=U#S33IZ=R!K2IF]\1D@
M*@I6;)=1P[3ICJ%,C5VTH^%^^N7(`)5NO*-SG)Y`QA_WK>PA8;TJ+X2)EV?3
MI.G"[*>WWDZ7\Q_8`@,?X8C9YSMNGQ.S79!10SGB!PGY<3)L+A>\T4NE3RCH
M@$<!]40^I5;'[)@>$KCW3*:VMQ")"FQ!"L?^:Y5K)WM]*CV<",@L38:E&'G;
MOH/\?B8-H-/5$-+1`SZ2O6RHY>T@2+Q"LM7T32<P'/M;:&`9G%<:2]0W95K<
M\.;8"EQMW`_,%NF[4)3]`F-BE^_T2,`,VV:G?3TUJ"IH\A@>7Y8:?5[I8HAX
M/S[+K[U+U);"B>&TTWB[]4K_N9HNW6P5\,!8G[BS*%\3<$#RM"O`6C#I1>3W
ME/8D\";AO1[@@M<^$07W%./W$^^MXKWV/QE?(SEU3GAF"?/TXQ_1N>#/)-;8
M<MT]?:XBQTJ%9'[+D!X/9^"U.Y*2A(8A/AJ\!K^V\)9>''<,=_M*GQ;D4XU&
M0EL_A7==:2!F6+.18],W)'0*A8VHHT@F\^+U@*=PAW&]_UDA'O3HL,)67*56
M:4QHF]DC7*EMI@?8=/,8;'O4[2W^#V!$L.(O\+@E"I[>5AF7V].]DSR0?>4E
MX,HXX%S-A'V'+F)3(0_FX[O!VNO&D/BL`T<!(LJ,(@3!S8LJ[><9CHP*Z!1N
M*W=F7"R-"ZZ_7"_NQ:8M=&RU\7`Z`<J>2YY>W2B\R7!YX(:UE+7?[1HLZ6?B
M_$Z<[I3[+!'?["<`BV0HHVNVZ$?*E!CLQ,Q'U$5$IQ?#B#+P)WE/$$-*'U<E
M%+7[;;^I\)F++WU`\\BL@X5,*B+]OLWQ&=W!,3*4_5Z0'R\2N/\];P>]W2<(
M-7Z3$YL3&6;-"*NTT2_Z1P=4">JZT,0Y$Q`L;RU@2\X!6>NA6D:5#HOIP#H]
MH)2I8$WFSU,1M9OC73J.1T1-YWD[%EH?1E*H#MV/[+5HSKGU.-%-'Z)QI=$\
MV>24Q6&KMA*-=L#[#I[2'0$86N)&8E/==`F@,S#5,`)(-KDT6A9IK-/%6OA@
M@FI$$#7G>&.Z!8[?8:F==P>HX>WF&.?(9V6^WM`J[CVD`L]9<&\6P_U?*WN`
MLI*_M*H;SP58M&#X!>*U*^J*XO@UT"&SIGH1%(K-=7DN@=HD6`S2EET,60JV
MI_\%)%6Q_^3CW_5`HQ;G084_7J0'F9DDH*%`SY*.D1BP`"D_QO=5,F?$-HAG
M_FP7H+LUTX`^%F-[SV(C'N*+AXE=&!+'OT$)RYGQ/HX,L8W(D%G9J=P!6+*$
M)F20)=%>9ZI).Z0`I'T/OT#SUR_:(O0U1*2-:,\D0S52^NI?HL69"POCNH&X
M_HXLQZB3EL-Z<)4.!<<BDJX3H"Q`L'&"RLO/%]17EV.5R@/$,%GYE#U(,Z'.
M6#]?M?@0VYB%WU2-4E:Z&9RN,"SCQAYJ70='?0`L5JC6GG#:1BG]DBCY;N)<
M>[;>JU-P]W=*RQG_KX;[>Y-0O.>_BS[M!=3Y#98EA`S8J/\1S=Z..*RC^;+U
M!.(#>E-V^?_+/M323Q,+EM-95M%CT#G[XO0FH/`.&`__EU<3\=+#>7?FR*NY
M9MA;$1+KD8?V@Y8XE7`(*;.N\KEF1]T4!OYS1+%#*S9&[0-#E"FRGA^\L[^A
M\76@2CMV<J_S92KW%;UO$.=R!3!P]OD.WD@*ZE(.;>H)8L]IMC;<YHPNH343
MY,JGKBM7M:!:S]$UJ7Y-/A'>Z]^7LV*Y.]N\MN_#%%%>)IH_A_:G46]+E.M;
MUA@I>99IP916P;7A48N3VF+;&!__1Q<QF8AU`XF-LJ./^6J+@+JCLICOF=I-
M.U"KKV.._JR/;P(````4=99=LD1P/P`!MPV`4```*JOOO;'$9_L"``````19
!6@``
`
end

I've also seen various programs that use the Braille Unicode characters for higher-resolution bitmap rendering, like mapscii, and I suspect that someone's probably written software to convert to that.