Is the Apple solution equally secure as a TrueCrypt using the same encryption?
Assuming that there is no "publicly known" security flaw within the file structure of the images created by said applications, the logical strength of said "container" would depend on the encryption algorithm utilized as well as the pass phrase implemented. Although, there are conspiracy theories surrounding TrueCrypt, they are beyond the scope of this question.
Is there any alternative that can easily be integrated into the OS to encrypt single files instead of whole disk images?
Yes, GnuPG, as stated by "mikeazo". You could generate a key-pair and encrypt your "sensitive" data using the public-key. You could write a script which would encrypt and decrypt, etc, all required files as necessary.
On the note of encryption algorithms, Blowfish, designed by Bruce Schneier, has the following notes based on cryptanalysis work:
There is no effective cryptanalysis on the full-round version of
Blowfish known publicly as of 2011. A sign extension bug in one
publication of C code has been identified. In 1996, Serge Vaudenay
found a known-plaintext attack requiring 28r + 1 known plaintexts to
break, where r is the number of rounds. Moreover, he also found a
class of weak keys that can be detected and broken by the same attack
with only 24r + 1 known plaintexts. This attack cannot be used against
the regular Blowfish; it assumes knowledge of the key-dependent
S-boxes. Vincent Rijmen, in his Ph.D. thesis, introduced a
second-order differential attack that can break four rounds and no
more. There remains no known way to break the full 16 rounds, apart
from a brute-force search. Bruce Schneier notes that while Blowfish is
still in use, he recommends using the more recent Twofish algorithm
instead.
~ Wikipedia
GnuPG can utilize Blowfish; Blowfish has also been included in the mainline GNU/Linux kernel since version 2.5.47. A list of products utilizing the Blowfish cipher may be found here.
On a side note, the key size utilized by the encryption algorithm (chosen by you) also plays a role in the overall security of said information. Generally, the larger the key (measured in bits {b}), the more computing power required to brute-force said pass phrase.
